Abstracts

Excessive Silencing of Orexin Neurons by Protons in a Mouse Model of Epilepsy and SUDEP

Abstract number : 3.160
Submission category : 3. Neurophysiology / 3F. Animal Studies
Year : 2018
Submission ID : 502754
Source : www.aesnet.org
Presentation date : 12/3/2018 1:55:12 PM
Published date : Nov 5, 2018, 18:00 PM

Authors :
Ted J. Warren, Creighton University School of Medicine; Timothy A. Simeone, Creighton University School of Medicine; and Kristina A. Simeone, Creighton University School of Medicine

Rationale: Orexin neurons within the lateral hypothalamic area (LHA) participate in various physiological functions and behaviors such as maintaining a wake state and autoresuscitation in response to changes in blood chemistry.  We previously found that orexin neurons contribute to sleep disorders and seizures in epileptic Kcna1-null (KO) mice, a model of temporal lobe epilepsy and SUDEP. Furthermore, we found that KO mice experience significantly more apneic-hypopneic (A-H) events which may contribute to early mortality. A-H events cause hypoxia, hypercapnia and acidification which initiate centrally-mediated autoresuscitation.  Repeated or intermittent hypoxia/hypercapnia has been reported to decrease orexin neuron activity as well as decrease the ability to autoresuscitate. Here, we tested the hypothesis that acidification reduces orexin neuron activity of KO mice to a greater degree than wild-type (WT). We also sought to determine if the orexin neurons’ chemoresponsiveness is intrinsic to the neurons themselves or relies on synaptic noise. Methods: Spontaneous activity was recorded extracellularly with a multielectrode array in an in vitro preparation of LHA slices. Single-unit waveforms were separated using principal component analysis and identified as belonging to principal cells (PCs) or interneurons based on spike symmetry versus spike length. Putative orexin neurons were identified by an increase in firing frequency in response to the A1R antagonist XCC. The pH of the aCSF was changed from pH 7.4 to pH 6.7 by adjusting the bicarbonate concentration. All data are reported as mean ± SEM. Results: Orexin neurons composed 52% and 80% of the WT and KO recorded PCs, respectively, with similar baseline firing frequencies of 1.72 ± 0.39 and 2.37 ± 0.88 Hz (p = 0.34). The firing frequency of most orexin neurons decreased in response to lowering pH to 6.7 (70% of WT: -11.7 ± 5% v.  75.5% of KO: -69.21 ± 10% decrease; p < 0.001). To test whether synaptic noise influences orexin responses to acidification, we eliminated synaptic activity with a high [Mg2+]e and low [Ca2+]e aCSF. In the absence of synaptic inputs, pH 6.7 produced a significantly greater reduction of spontaneous activity of WT orexin neurons (p = 0.01), whereas firing of orexin neurons from KO mice was reduced to a similar degree with or without synaptic inputs (p = 0.75). Conclusions: These results indicate that acidification dampens the intrinsic firing properties of orexin neurons. In WT slices, synaptic inputs attenuate the effects of acidification on orexin neurons exemplifying the importance of network interactions. In this light, the lack of effect of synaptic input on the significantly greater pH response of orexin neurons from epileptic KO mice may be due to a loss of an excitatory synaptic regulation and warrants further study. Overall, our data suggests that acidification due to an A-H event would significantly reduce the activity of orexin neurons in KO mice and dampen their participation in autoresuscitation, a situation that would be detrimental during the sequence of seizure-induced apnea and SUDEP. Funding: Not applicable